1. Field of the Invention
This invention relates to the field of semiconductor devices. More particularly, the invention relates to a method and apparatus for resolving conflicts between phase shifted structures and non-phase shifted structures during the definition of masks to be used in optical lithography processes for manufacturing integrated circuit devices.
2. Description of the Related Art
Semiconductor devices continue to be produced at reduced sizes as optical lithography processes have evolved. Techniques such as phase shifting have been developed to assist in the production of subwavelength features on the integrated circuits (IC) using optical lithography processes. Subwavelength features are features that are smaller than the wavelength of light used to create circuit patterns in the silicon. More generally, phase shifting can be used to create features smaller than a minimum realizable dimension for the given process.
Through the use of phase shifting masks, such subwavelength features can be efficiently produced. (Note, that the term “mask” as used in this specification is meant to include the term “reticle.”) One approach to producing a phase shifting mask (PSM) is to use destructive light interference caused by placing two, out of phase, light transmissive areas in close proximity in order to create an unexposed region on a photoresist layer of an IC. If that unexposed area is then protected from exposure when a trim mask is used to expose the remaining field (thus causing definition of the remaining structure), the resultant IC will include subwavelength features created by the PSM.
One approach to preparing an IC for production using PSMs is for one or more features of the IC to be identified for production using PSMs. For example, a designer might identify one or more particular features for production using the PSM, e.g. to define the identified gates (or other features) at subwavelength sizes.
A portion of a design layout 100 for a layer in an IC is shown in FIG. 1. Several distinct portions of the design layout are identified, particularly a field polysilicon 104, a gate 102 and a structure 106. In this example, the gate 102 is identified as “critical”, e.g. specified for production using a PSM.
A phase shifting mask 200 for defining the gate 102 is shown in FIG. 2. Light transmissive region 202 and light transmission region 204 are out of phase with one another, e.g. light through one is at phase 0 and the light through the other at phase π. These light transmissive regions are sometimes referred to both individually and collectively as phase shifters (the meaning will be apparent from usage). Additionally, the light transmissive regions are sometimes referred to as phase shifting areas. Also shown on FIG. 2 is an outline 206 of where the structure 106 is relative to the openings for the phase shifters. Particularly, the outline 206 is overlapped by the light transmissive region 204.
Thus, FIG. 2 generally illustrates one example of the class of problems to be addressed. In particular, it is generally preferable to make the phase shifters (e.g. the light transmissive region 202 and the light transmissive region 204) relatively wide compared to the wavelength of the light (λ). For example, some PSM processes attempt to make the total width of the phase shifters and the protective area between them approximately 3λ. However, it is unacceptable to allow the phase shifters to directly overlap areas where there are non-phase shifted structures (e.g. overlap area 208).
Similarly, because of optical effects, e.g. light traveling under the phase shift mask, due to mask misalignment between the PSM and the trim mask, etc., some non-phase shifted structures in close proximity to where phase shifters are placed on the PSM may become exposed at the time the PSM 200 is used.
Accordingly, what is needed is a method and apparatus for resolving conflicts resulting from placement of phase shifters in close proximity to structures being produced other than by phase shifting. Additionally, both a PSM and a trim mask that can produce ICs with subwavelength structures that are in close proximity to at or above wavelength structure are needed.